ABSTRACT Although in general, the larger the primary tumor the greater the likelihood that it will metastasize or already has metastasized, this is not always the case. Many small breast cancers develop metastasis and have a discouraging outcome. Characterizing genes that drive these tumors' rapid progression may identify novel biomarkers to help clinicians guide current treatments, and may offer novel therapeutic targets. We have discovered that EZH2 is overexpressed in 55% of invasive breast carcinomas and is an independent tissue biomarker of poor outcome. EZH2 is a Polycomb group protein responsible for maintaining cell identity through successive generations of cells. During the previous funding cycle, we have demonstrated that EZH2 protein is upregulated during progression from normal breast to ductal carcinoma in situ, the precursor of invasive carcinoma, to invasive carcinoma, being highest at the metastasis. EZH2 triggers invasion and regulates breast cancer growth in vivo and in vitro. Our lab has provided the first mechanistic link between EZH2 and BRCA1. EZH2 controls the intracellular distribution of BRCA1 on breast cells and regulates the transition between G2 and mitosis and cell proliferation in a BRCA1-dependent manner. A major accomplishment of our laboratory has been the development of a mammary specific EZH2 transgenic mouse model. EZH2 transgenic mice develop epithelial intraductal hyperplasia with downregulation of nuclear BRCA1 protein and histological features recapitulating human disease. Based on this body of work, the central hypothesis of our competing renewal is that EZH2 overexpression in the mammary gland induces hyperplasia by regulating BRCA1 protein and function. We further hypothesize that additional specific oncogenic events in the setting of EZH2 overexpression, trigger the rapid development of carcinomas that are highly aggressive from the outset. The specific aims are: Aim 1. To investigate if EZH2 overexpression in the mammary gland of transgenic mice accelerates tumor development and metastasis using well-characterized models of breast tumorigenesis that recapitulate EZH2 overexpressing human invasive breast carcinomas; Aim 2. To determine the mechanism by which EZH2 regulates BRCA1 protein and BRCA1-mediated tumor suppressor functions; Aim 3. To evaluate the clinical usefulness of detecting EZH2 alone or in combination with BRCA1 (a) as a tissue biomarker of increased breast cancer risk, and (b) as a biomarker able to identify which small invasive carcinomas have high metastatic potential. We will investigate the effect of EZH2 overexpression on BRCA1 in humans using unique resources consisting of benign breast tissues of women who subsequently developed breast cancer, and a cohort of invasive carcinomas of the breast exclusively of Stages 1 and 2 (< 2 cm) with 10 years of follow-up. These studies will provide better diagnosis, more accurate predicting of poor prognosis and the potential to develop new therapies.